Polarized light is created from most lamp assemblies using a flat window which absorbs one linear polarization state while passing the other. As an alternative, many device concepts are known within the art which do not absorb but actually reflect the unwanted polarization state using repeating prism structures. This reflected light can then be rotated or otherwise converted within the lamp assembly so that it passes through the prismatic device on future attempts, thereby delivering fully polarized light without the absorption losses of conventional polarizers.
However, such prismatic device concepts are difficult to manufacture because they require acute prism apex angles (90 degrees or less) and further require materials having a high index of refraction. Many such concepts further rely substantially on the conventional polarization properties of light incident at a 45 degree angle to a refractive boundary. These polarization properties are optimum at the Brewster angle which is dependent upon the indices of refraction of the adjoining media. For light passing from air into a material with an index of refraction of about 1.6, the associated Brewster angle is approximately 58 degrees, so that the above described prismatic device with a 45 degree angle to the refractive boundary would not achieve suitable efficiency unless expensive coatings are applied.
In accordance with Maxwell's Equations, at the Brewster angle, reflected light is polarized perpendicular to the plane of incidence, i.e. .sigma.-polarization, and the transmitted/refracted light is mostly polarized parallel to the plane of incidence, i.e. .pi.-polarization. The Brewster angle .theta..sub.B is given by .theta..sub.B =tan.sup.-1 (n.sub.T /n.sub.l) where n.sub.T is the index of refraction of the medium of transmittance/refraction and n.sub.l is the index of refraction of the medium of incidence/reflection.
U.S. Pat. No. 5,559,634 to Weber discloses and claims a retroreflecting polarizer comprising a plurality of thin film stacks coated onto substrates having structured surfaces comprising a linear array of isosceles prisms having sides which make an angle in the range of 40-50 degrees, which is relatively expensive and difficult to produce because of the relatively large angle of the associated structured surface, and the need for a plurality of different coatings necessary to satisfy the MacNeille conditions whereby the reflection coefficient for .pi.-polarization is essentially zero at each film interface.